OCR Specification focus:
‘Identify independent, dependent, and control variables, and specify how each control variable will be monitored or kept constant throughout the investigation.’
Understanding how to identify and control variables is essential for designing valid scientific experiments. It ensures results are reliable, comparable, and accurately reflect the relationships between measured quantities.
Understanding Variables
In experimental physics, a variable is any factor, condition, or quantity that can change or be changed during an experiment. Variables are categorised based on their role in the investigation.
Schematic showing how an independent variable is manipulated, the dependent variable is measured, and control variables are held constant between groups. This directly mirrors the OCR emphasis on identifying and controlling variables. Note: the image also mentions control and experimental groups; this is acceptable background but not required by the sub-subtopic. Source
Independent Variable
The independent variable is the quantity that the experimenter deliberately changes or manipulates to observe its effect on another variable.
Independent Variable: The factor that is intentionally altered in an experiment to observe its effect on the dependent variable.
For example, in an investigation of how current affects the heating of a resistor, the current is the independent variable.
Dependent Variable
The dependent variable is the factor that is measured or observed in response to changes in the independent variable.
Dependent Variable: The factor that is measured or recorded as it responds to the changes made to the independent variable.
In the same resistor experiment, the temperature of the resistor is the dependent variable, since it depends on the current passing through it.
Control Variables
Control variables are quantities that must be kept constant to ensure the experiment remains a fair test. They prevent external factors from influencing the dependent variable.
Control Variable: A quantity that could influence the dependent variable and must therefore be kept constant or monitored throughout the experiment.
In the resistor heating example, control variables might include ambient temperature, resistance of the resistor, and duration of current flow.
Identifying Variables in an Investigation
Identifying variables correctly is the first step in designing a valid experiment. The process typically involves:
Defining the aim of the experiment clearly.
(e.g. “To determine how the length of a wire affects its resistance.”)
Determining what will be changed — this is the independent variable.
Deciding what will be measured — this is the dependent variable.
Recognising potential influences — these become the control variables.
When identifying variables:
Use precise, measurable quantities (e.g. temperature in °C, time in s).
Ensure that only one independent variable is changed at a time.
Make sure that all other variables remain constant to maintain validity.
Controlling Variables
Importance of Control
Controlling variables allows an experiment to isolate the effect of the independent variable. Without control, results may be invalid or misleading.
Uncontrolled variables introduce uncertainty and bias, making it difficult to draw accurate conclusions. Control ensures that any observed effect can confidently be attributed to the independent variable.
Methods of Control
Different strategies are used depending on the type of variable being controlled:
Environmental control: Maintain constant temperature, humidity, and lighting conditions.
Example: Use a thermostatically controlled water bath in temperature-dependent experiments.
Laboratory water bath used to maintain a constant temperature, ensuring environmental conditions are controlled during measurements. This apparatus exemplifies practical control of confounding variables to preserve validity. The image shows the bath body and controls; no extra syllabus content is introduced. Source
Instrumental control: Use the same measuring equipment and ensure it is calibrated correctly.
Example: Use the same voltmeter for all voltage measurements.
Procedural control: Keep methods consistent, such as timing intervals or the sequence of steps.
Example: Stir solutions for the same duration each time.
Material control: Use identical materials or samples where possible.
Example: Use wires made from the same metal and with the same cross-sectional area.
Mathematical control: When a variable cannot be kept constant, measure it and apply corrections during analysis.
Example: Record ambient temperature and adjust results accordingly.
Monitoring Control Variables
Even with good planning, some control variables cannot be perfectly fixed. In such cases, they must be monitored during the experiment.
Monitoring Techniques
Record readings of environmental factors at regular intervals.
Use sensors or data loggers for continuous monitoring of quantities like temperature or pressure.
Compare repeated trials to check consistency of controlled conditions.
Monitoring allows you to:
Identify fluctuations that may affect data reliability.
Apply appropriate corrections or note potential limitations.
Example of Variable Relationships
In most experiments, the relationship between variables can be expressed mathematically as a functional dependence:
EQUATION
—-----------------------------------------------------------------
General Relationship: Dependent Variable ∝ Independent Variable
∝ means “is directly proportional to” — the dependent variable changes in proportion to the independent variable when other factors are constant.
—-----------------------------------------------------------------
For example, Ohm’s Law (V = IR) describes the relationship between potential difference and current for a resistor at constant temperature.
Figure labelled “Test circuit for evaluating Ohm’s law.” The ammeter is in series and the voltmeter is across the resistor, making clear which quantity is changed and which is measured. This supports variable identification and highlights the need to control temperature for validity. Source
Here:
Independent variable: current (I)
Dependent variable: potential difference (V)
Control variable: temperature of the resistor
This illustrates the importance of maintaining constant temperature; if it changes, resistance varies, breaking the proportionality.
Ensuring Validity and Reliability
Validity
An experiment is valid if it measures what it is intended to measure.
Controlling variables ensures that only the intended relationship is tested.
To ensure validity:
Keep all other influencing factors constant.
Use appropriate apparatus that can achieve the required precision.
Follow consistent experimental procedures.
Reliability
Reliability refers to the consistency of results when an experiment is repeated.
Reliability is improved by:
Taking multiple readings for each independent variable value.
Repeating experiments under the same conditions.
Averaging results to minimise random error.
Both validity and reliability depend on careful identification and control of variables. Poor control leads to systematic errors, while inconsistent monitoring results in random errors.
Summary of Key Practices
Identify independent, dependent, and control variables clearly before experimentation.
Change only one variable at a time.
Control all other relevant factors as much as possible.
Monitor variables that cannot be fixed, and record their influence.
Maintain consistency in equipment, environment, and procedure.
Evaluate whether control methods were effective when analysing results.
Effective control of variables underpins the accuracy, validity, and credibility of every experimental investigation in physics.
FAQ
If a control variable changes significantly, it introduces systematic errors that shift all results in one direction. This means the data no longer accurately reflects the true relationship between the independent and dependent variables.
For example, in an electrical experiment, if the temperature of a resistor increases, its resistance changes, affecting every measurement taken.
To detect such problems, students should monitor control variables throughout the experiment and note any variations that could influence the outcome.
In A-Level Physics, only one independent variable should be changed deliberately during a single experiment.
If multiple variables are altered, it becomes impossible to determine which one caused the observed effect, leading to invalid conclusions.
When several factors might influence results, the investigation should be divided into separate experiments, each isolating one independent variable while keeping others constant.
Human error can cause inconsistency when variables are manually controlled or measured. Examples include misreading instruments, inconsistent timing, or irregular sample preparation.
To reduce this effect:
Use digital sensors or data loggers to maintain precision.
Ensure the same operator performs all measurements.
Follow standardised procedures and repeat readings.
Minimising human involvement in variable control enhances the reliability and objectivity of results.
Stating how a variable is monitored demonstrates awareness of potential change and ensures quantitative justification for control.
For example, saying “temperature is kept constant” is insufficient. Instead, specify that a thermometer or temperature probe is used to verify constancy within a certain range.
This precision allows evaluators to judge whether the experimental method can truly maintain valid and reliable conditions.
Controlling a variable means actively maintaining it at a specific value or ensuring it remains unchanged.
Standardising refers to keeping experimental procedures, materials, or instruments identical across all trials so that variation is minimised indirectly.
For instance, using the same stopwatch, operator, and measurement method for every trial is standardisation, while maintaining the same temperature using a thermostat is control.
Both approaches are vital for ensuring comparability and fairness in experimental results.
Practice Questions
Question 1 (2 marks)
A student investigates how the extension of a spring depends on the force applied to it.
(a) Identify the independent, dependent, and one suitable control variable in this investigation.
Mark scheme:
Independent variable: Force applied to the spring (1 mark)
Dependent variable: Extension (length increase) of the spring (1 mark)
Control variable examples (no mark required beyond identification of the above two): Temperature, material of the spring, or spring used must remain constant.
Question 2 (5 marks)
A student designs an experiment to determine how the resistance of a wire depends on its length.
Describe how the student should identify and control variables to ensure the results are valid and reliable. In your answer, you should:
Name the independent, dependent, and control variables.
Explain how each control variable should be maintained or monitored.
Describe how reliability could be improved.
Mark scheme:
Independent variable: Length of the wire (1 mark)
Dependent variable: Resistance (calculated from potential difference and current readings) (1 mark)
Control variables (maximum 2 marks):
Material and cross-sectional area of the wire kept constant by using the same type and gauge of wire. (1 mark)
Temperature controlled or monitored by keeping current low or allowing the wire to cool between readings. (1 mark)
Reliability improvement (1 mark):
Take repeated readings for each length and calculate mean values, or plot a graph of resistance against length to identify anomalies and assess consistency.
